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Region.md

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Region Class

Overview

Region represents a bounded planar area with physical properties (Area, Perimeter, Centroid, Inertia). Regions are created from closed loops of curves (lines, arcs, polylines) and form the basis for creating complex 2D shapes that can be extruded into Solid3d objects.

Namespace

Autodesk.AutoCAD.DatabaseServices

Inheritance Hierarchy

System.Object
  └─ RXObject
      └─ DBObject
          └─ Entity
              ├─ Region

Key Properties

Property Type Description
Area double Total area of the region.
Perimeter double Total length of boundaries.
Normal Vector3d Normal vector of the region plane.
AreaProperties RegionAreaProperties Complex physical properties.

Key Methods

Method Return Type Description
CreateFromCurves(DBObjectCollection) DBObjectCollection Static method to create regions from curves.
BooleanOperation(BooleanOperationType, Region) void Union, Subtract, Intersect.
GetAreaProp(Point3d, Vector3d, Vector3d) void Calculates area properties relative to axes.

Code Examples

Example 1: Creating a Region from a Circle

using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTableRecord btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;
    
    // Define input curve
    Circle circ = new Circle(Point3d.Origin, Vector3d.ZAxis, 5.0);
    DBObjectCollection curves = new DBObjectCollection();
    curves.Add(circ);
    
    // Create Region
    DBObjectCollection regions = Region.CreateFromCurves(curves);
    
    if (regions.Count > 0)
    {
        Region reg = regions[0] as Region;
        btr.AppendEntity(reg);
        tr.AddNewlyCreatedDBObject(reg, true);
    }
    
    // Dispose collection (but NOT the region we added to DB)
    regions.Dispose();
    tr.Commit();
}

Example 2: Boolean Intersection

// Two overlapping regions
Region r1 = ...;
Region r2 = ...;

// Intersect: Result is the common area
r1.BooleanOperation(BooleanOperationType.BoolIntersect, r2);

// r1 is now the intersection
// r2 is invalidated
r2.Dispose();

Example 3: Calculating Area

public void ShowArea(Region reg)
{
    ed.WriteMessage($"\nArea: {reg.Area}");
    ed.WriteMessage($"\nPerimeter: {reg.Perimeter}");
}

Example 4: Creating Complex Shape (Subtracted)

// Create square
DBObjectCollection p1 = new DBObjectCollection();
p1.Add(new Polyline(4)); // ... assume 10x10 square ...
Region outer = Region.CreateFromCurves(p1)[0] as Region;

// Create circle
DBObjectCollection p2 = new DBObjectCollection();
p2.Add(new Circle(center, normal, 2.0));
Region inner = Region.CreateFromCurves(p2)[0] as Region;

// Subtract hole from square
outer.BooleanOperation(BooleanOperationType.BoolSubtract, inner);
inner.Dispose();

// Result: Square with a hole

Example 5: Exploding a Region

DBObjectCollection parts = new DBObjectCollection();
region.Explode(parts);

// Result is usually the boundary curves (Lines, Arcs, Splines)
foreach (Entity e in parts)
{
    // ...
}

Example 6: Checking Coplanarity (Prerequisite)

// Region.CreateFromCurves fails if curves are not coplanar
// or not closed.
// Always validate input curves.

Example 7: Extruding to Solid

Region reg = ...;
Solid3d sol = new Solid3d();
sol.CreateExtrudedSolid(reg, Vector3d.ZAxis * 5.0, new SweepOptions());

Example 8: Mass Properties Analysis

Plane plane = new Plane(Point3d.Origin, Vector3d.ZAxis);
Solid3dMassProperties props = reg.GetMassProperties();

ed.WriteMessage($"\nCentroid: {props.Centroid}");
ed.WriteMessage($"\nMoments: {props.PrincipalMoments}");

Best Practices

  1. Input Validation: CreateFromCurves is strict. Curves must form a closed loop, be planar, and not self-intersect.
  2. Disposal: Like Solid3d, Regions wrap native ACIS entities. Dispose if not persistent.
  3. Boolean Ops: Operands must be on the same plane.
  4. Collections: CreateFromCurves returns a collection because one set of curves might result in multiple disjoint regions (e.g., two circles). Handle all returned regions.

Related Objects

  • Solid3d - 3D equivalent.
  • Hatch - Often uses boundaries similar to Regions.

References